He essentially got his opportunity because Brook & McDermott were missing , so probably is their 7th batsmen.
Given Brook & McDermott are now available they both play.
Possibly but highly unlikely that Wade , Short , Handscomb or David lose their spot.
I guess they could play 7-4 and rely on David/Short to be the 5th bowler.
Brook could well be another Jacks
They could also rotate 7 into 6 each match depending on matchups , opposition & venue